Howard Weinstein

A method of simultaneous dual-radionuclide cardiac imaging with technetium 99m and thallium 201. I: Analysis of interradionuclide crossover and validation in phantoms.

BackgroundSimultaneous dual-radionuclide technetium 99m/thallium 201 scintigraphy can potentially produce perfectly aligned stress and rest images in less time than conventional protocols. However, interradionuclide crossover limits diagnostic accuracy. Accordingly, we evaluated99mTc and201Tl crossover in line and heart phantoms.Methods and Results99mTc crossover in the201Tl imaging window constituted as much as one half of the counts in the99mTc window, varied significantly with attenuation, and was spatially incoherent.201Tl crossover was relatively small, less variable, and spatially similar to the primary image. Based on these findings, the following simultaneous dual-radionuclide99mTc/201Tl method was developed, and validated in line and heart phantoms. The99mTc source is imaged first into dual201Tl/99mTc windows, followed by201Tl administration and dual-radionuclide imaging. The single-radionuclide99mTc image in the201Tl window is count-normalized for acquisition time and then subtracted from the dual-radionuclide201Tl image to specifically correct for99mTc crossover. Image quality of the corrected dual-radionuclide201Tl images approached their single-radionuclide counterparts. Correction for201Tl crossover was relatively unimportant.ConclusionSimultaneous dual-radionuclide99mTc/201Tl myocardial scintigraphy is feasible with99mTc crossover correction specific to each acquisition. The proposed dual-radionuclide99mTc/201Tl method and the principles on which it is based can be applied to a broad range of dual-radionuclide pairs.

Myocardial uptake of thallium 201 and technetium 99m-labeled sestamibi after ischemia and reperfusion: comparison by quantitative dual-tracer autoradiography in rabbits.

BackgroundMyocardial scintigraphy with99mTc-labeled sestamibi (99mTc-sestamibi) or201Tl is used to assess regional perfusion in acute coronary syndromes associated with metabolic or functional abnormalities, such as acute coronary thrombosis with reperfusion and ischemia at rest. However, the initial uptake of these agents may be affected by a recent ischemic insult because the myocardial retention of these tracers depends on cellular metabolism.Methods and ResultsAccordingly,99mTc-sestamibi and201Tl were injected simultaneously in rabbits after transient brief (10 to 15 minutes, group I) or prolonged (45 to 60 minutes, group II) coronary occlusion. Accumulated subendocardial and subepicardial99mTc-sestamibi and corresponding201Tl activity were determined from autoradiographs of 30 µm short-axis slices comounted with serial tissue standards. Circumferential99mTc-sestamibi and201Tl activity profiles closely overlapped in both groups. The initial global and segmental myocardial activity per unit blood flow within the ischemic zone did not differ from unity for either tracer regardless of the duration of the ischemic insult. The initial myocardial uptake of both99mTc-sestamibi and201Tl after an acute ischemic insult reflected predominantly coronary blood flow, independent of myocardial viability.ConclusionsThus this study supports the use of both99mTc-sestamibi and201Tl as perfusion probes in acute coronary syndromes characterized by acute occlusion and reperfusion.

Rapid redistribution of teboroxime

Teboroxime, a new technetium-99m-labeled myocardial perfusion tracer, possesses rapid myocardial kinetics. Whereas this agent is routinely imaged after separate stress and rest injections, experimental data suggest that teboroxime may rapidly redistribute in the myocardium. Accordingly, we assessed 68 exercise teboroxime scintigrams in which immediate poststress, early delay (5 minutes) and rest images were acquired. Studies were categorized visually as ischemia, infarct or normal based on conventional stress-rest comparison. They were then evaluated for rapid teboroxime redistribution by comparing the stress and early delay images. Quantitative analysis was then performed on 537 myocardial segments. Segments were grouped as ischemia, infarct or normal based on stress-rest comparison, and the degree of normalization of stress-induced defects in the early delay images was determined for each group. Rapid teboroxime redistribution was observed in 20 of 46 scintigrams (48%) considered ischemic, and in 2 of 7 and 2 of 15 scintigrams deemed infarct and normal, respectively. The mean segmental intensity ratio (defined relative to the opposite segment) improved from 0.79 at stress to 0.88 at early delay (p < 0.005) in the group with ischemia and from 0.83 to 0.87 in the group with infarction. The most likely explanation for rapid redistribution of teboroxime is differential washout from the myocardium between areas of disparate flow. It is concluded that rapid redistribution of teboroxime occurs within 5 minutes of a stress injection, giving rise to potentially useful clinical information. Thus, teboroxime imaging should be completed expeditiously to detect areas of relative hypoperfusion.

Comparison of teboroxime and thallium for the reversibility of exercise-induced myocardial perfusion defects☆

To determine the optimal technique for the scintigraphic detection of exercise-induced myocardial perfusion defects, we compared teboroxime scanning to both stress/redistribution thallium imaging and the thallium reinjection method following exercise in 35 patients. The overall concordance for the presence of a perfusion defect between teboroxime and thallium scanning was 91% (p < 0.01) and 89% when teboroxime was compared with stress/reinjection thallium imaging (p < 0.01). More segments per scan with fixed defects were observed with redistribution imaging than with teboroxime or thallium reinjection (2.9 vs 2.0 vs 1.9; p < 0.02). Additionally, more transient defects were present with teboroxime than thallium, but less than with reinjection imaging. One half of the 52 fixed perfusion abnormalities on stress/redistribution thallium imaging demonstrated reversibility with both teboroxime imaging and thallium reinjection scanning, but less than 50% of these segments were concordant. Teboroxime allows for improved detection of reversible perfusion defects compared with stress/redistribution thallium scanning, but more ischemia is noted with thallium reinjection. The variation in the detection of segmental ischemic defects between teboroxime scintigraphy and thallium reinjection scanning probably reflects different physiologic properties and imaging protocols of these perfusion agents.